September 22, 1921] 



NATURE 



131 



wide range in size and price, 'and many object-glasses, 

 eve-pieces, and other accessories are also listed. Sur- 

 veying instruments, particularly theodolites and levels, 

 are well represented. In the section devoted to tele- 

 scopes, a number of second-hand instruments, both 

 reflectors and refractors, on equatorial and altazimuth 

 mounts, are offered for sale. The list includes a 12-in. 

 reflector and a 12-in. and a 7i-in. refractor, as well 

 as several smaller instruments, object-glasses, eye- 

 pieces, sidereal clocks, and other astronomical appara- 

 tus. .\ series of lantern-slides has been prepared to 

 show the appearance of the bright line spectra between 

 the limits 4000-7000 A.U. of the commoner elements 

 with normal dispersion. Slides of twenty-seven ele- 

 ments are now available ; they should be of consider- 

 able service to science teachers. Other features of 

 the catalogue are the sections dealing with cameras 

 and other photographic apparatus and books ; the 

 latter contains, among a number of useful text-books 



and series of scientific periodicals, vols. 28 to 104 of 

 Natirf. 



The Cambridge University Press is publishing the 

 three following books in the autumn : — " New Mathe- 

 matical Problems," by Major P. A. MacMahon ; 

 "Series Spectra," by Dr. Norman R. Campbell; and 

 •"Weather Prediction by Numerical Process," by L. F. 

 Richardson. The first-named will be problems based 

 on the permutations and combinations of elementary 

 geometrical shapes ; the second will be the first of the 

 supplementary chapters to the author's " Modern Elec- 

 trical Theorv," to which allusion has already been made 

 in Nature (February 24, p. 842) ; and the third em- 

 bodies a scheme of weather prediction, resembling the 

 process employed in the production of the Nautical 

 Almanack. At the close of the present year the same 

 publishers will issue "Alternating Currents," in two 

 parts, by C. G. Lamb. It is intended as a guide to the 

 student attending a three-term course on the subject. 



Our Astronomical Column. 



September Meteors. — Mr. \V. F. Denning writes : — 

 '■ An excellent series of abundant observations were 

 obtained during the first ten days of the present 

 month by Miss A. Grace Cook and Mr. J.. P. M. 

 Prentice at Stowmarket, and several hundred 

 meteor paths were carefullv recorded, from which a 

 number of interesting radiant points were derived. 

 These include various systems which have been well 

 observed in past years, and several which apparently 

 represent new showers. 



"An active radiant of Orionids from ()i^°+S^° was 

 detected on September 1-3, which seems to have 

 escaped previous observation ; and among the old 

 streams we observed were the e .Arietids, o Cas- 

 siopeids, a-/3 Perseids, and t Aurigids. 



"Fireballs are usually very frequent in September, 

 and the present month has proved no exception. 

 Several brilliant meteors from Capricornus were ob- 

 served on September 6, 7, and 14. On September 7 

 and 8 large meteors were seen from a radiant in 

 .Auriga, and on September to two brilliant objects 

 were recorded, possibly from a radiant near a Cvgni. 

 The times of the two latter were at 9.10 and 11.40 

 G.M.T., and further observations of these various 

 objects would be valuable. This display of Cvgnids is 

 a long-continued one, and was noted as specially 

 active on September 12, iqi8, as seen from Bristol." 



Iomsation in- Stellar .\tmospheres. — Dr. M. N. 

 Saha has published an important series of papers on 

 this subject in Phil. Mag. (vols. 40 and 41) and in 

 Proc. Roy. Soc.. gq A (1921). A useful summary and 

 critique of these is given bv Mr. E. A. Milne in 

 Observatory for September. ' The work consists of 

 two parts : the study of the conditions of ionisation 

 by the formulae of physical chemistry, and an en- 

 deavour to explain some of the features of solar and 

 stellar spectra in the light of the results. Taking 

 calcium as an example, the percentage of ionisation 

 at different temperatures and pressures is tabulated ; 

 the table indicates that at the surface of the sun both 

 normal and ionised atoms of calcium should be plenti- 

 ful, and, in fact, the g, H, K lines are all present. 

 On the other hand, at great heights above the photo- 

 sphere the pressure is small and ionisation almost 

 complete; accordingly, only the enhanced lines H, K 

 are visible here, g not being traced bevond a height of 

 5000 km. Analogous results are given for several 

 other elements, and hope is held out that the method 

 NO. 2708, VOL. I08I 



may eventually afford an indication of the pressure 

 at various heights above the photosphere. 



Lessons are also deduced from the progressive appear- 

 ance and disapjjearance of certain lines as we pass 

 along the series of stellar spectra from M to O. Dr. 

 Saha determines the temperature of each tyf)e, his 

 values ranging from 23.000° Oa, 18,000° Bo, to 5000° 

 Ma, and 4o<io° Md. These are in fair accord with 

 those of Russell, Wilsing, and Scheiner, but slightly 

 higher on the .average. 



It will be remembered that several astronomers have 

 suggested that, the supply of gravitational energy 

 being insufficient to maintain their output, the stars 

 are drawing on the energy of the atom. It is likely, 

 therefore, that atomic chemistry will play an important 

 part in the astronomy of the future. 



Variable Stars. — Observations of seventy-two 

 well-known variable stars, the R.A.'s of which 

 range from 5h. 21m. to 24h., were made by 

 Prof. \'ojtech Safari'k at Prague between the 

 years 1877 and 1894. They are reproduced .in great 

 detail bv Prof. Ladislav Pracka in a publication re- 

 cently received (' Untersuchungen uber den Licht- 

 wechsel .\lterer Veriinderlicher Sterne. Nach den 

 Beobachtungen von Prof. Dr. Vojtech Safafi'k." 

 Vol. 2, pp. iii+i8o. Prag : Fr. Rivnac, 1916). 

 The magnitudes of the comparison stars are discussed 

 and compared with all available authorities; the 

 differences of magnitude between them and the 

 variables are given in full, and the nature of the light 

 curve, with the dates of ma.ximum and minimum, is 

 discussed in all cases where the observations suffice 

 for the purpose. There are also many estimates ot 

 colour, on Schmidt's numerical scale, which represents 

 white by o, yellow by 4. orange by 7, red by 9 to 10. 

 The long-period variables in this volume are without 

 exception orange or red ; eleven of them have colour- 

 estimates extending beyond 9, and one star, S Cephei. 

 has a colour-estimate of 10. 



The following stars have especially long and full 

 series of obser\ations : R Leonis, R Camelopardi, 

 R Draconis, R Aquilae, L' Cvgni, V Cygni. S Cephei. 

 Observations of two novae are included in the volume. 

 Nova Aurigae fell from 5m. to iim. in a few weeks 

 early in 1892, then revived to 9-2m. early in 1893, 

 being g-gm. at the ^nd of that year. Nova T Coronae 

 appeared to remain steady at 9-2m. during the years 

 1886 to 1894. 



